286 VERTEBRATE ZOOLOGY 



the anus and above the urogenital aperture is the inguinal space, 

 a hairless depression into which the inguinal glands open. 



Exercise 11. Make a semidiagrammatic sketch of the genital organs. 



The Vascular System {Continued) ; the Caval Veins. We shall 

 now begin the study of the veins which carry venous blood directly 

 to the heart. Three groups of these veins are present: (i) the 

 right and left precaval veins and their branches, which bring blood 

 from the forward part of the body; (2) the postcaval vein and its 

 branches, which bring blood from the hinder part of the body ; 

 and (3) the coronary veins, which bring blood from the walls of 

 the heart itself. 



The Postcaval System. The anterior, thoracic portion of the 

 postcaval vein will be observed later, when the organs of the 

 thoracic cavity are studied. Its abdominal portion arises in 

 the hinder part of the abdominal cavity and passes forward near 

 the median plane, between the kidneys, to the right lateral lobe 

 of the liver, through which it passes ; it then pierces the diaphragm 

 and enters the thoracic cavity. 



Press the liver backward and observe the large postcaval vein 

 passing through the diaphragm from the liver. Then press the 

 liver forward and note where the vein enters it from behind. 

 Follow the postcaval to its hinder end, where it will be seen to be 

 formed by the union of two veins, the internal iliacs ; a short dis- 

 tance in front of these veins it is joined by a pair of larger veins, 

 the external iliacs. Trace the external iliac vein, but without in- 

 juring either the artery which accompanies it or the white nerves 

 of the lumbosacral nerve plexus. It receives the vesical vein— 

 which brings blood from the bladder and, in the female animal, 

 from the uterus also— and the inferior epigastric, which runs for- 

 ward along the rectus abdominis muscle and anastomoses with 

 the internal mammary ; finally, as the femoral vein, it continues 

 along the inner side of the leg. The internal iliac— usually the left 

 one— receives the small caudal vein from the tail. 



Trace the postcaval vein and its branches from the external 

 iliacs forward to the diaphragm, being careful not to injure the 

 arteries which accompany them. As the vein passes forward from 



